1. Field of Invention
This invention relates to a lock-up control system for a torque transmission device with a lock-up clutch that is arranged between a drive source and a drive wheel of a vehicle that has at least a motor or a motor generator in its drive source.
2. Description of Related Art
In a vehicle that has an engine, fuel is combusted within the engine, generating heat energy, and this heat energy is converted to mechanical energy (drive force) to drive the vehicle. With an engine, the driving region at which combustion efficiency is good and high torque can be obtained is limited to a relatively narrow r.p.m. (revolution per minute) range. Therefore, in a vehicle that uses an engine as the drive source, the engine r.p.m. and engine output torque are changed by a transmission depending on a driving condition, and transmitted to the vehicle wheel(s).
Incidentally, in recent years, hybrid vehicles have been proposed that have a different type of drive source, in particular, a motor generator, with objects of conserving the fuel which drives the engine, reducing the noise due to engine revolution, and reducing the exhaust gas that is generated through combustion of the fuel. It is proposed, in these hybrid vehicles, too, to use a transmission and a torque converter with a lock-up clutch between the drive source and the drive wheel(s), the same as in a vehicle that has only an engine as a drive source. For example, there is such a system disclosed in Japanese Laid-Open Application No. 8-168104.
Incidentally, a hybrid vehicle provided with a motor or motor generator as described above can run using the motor or motor generator as a drive source, and since this motor or motor generator has the advantage of having virtually no torque fluctuation as compared to an engine that combusts fuel, it is possible to control the lock-up clutch of a torque converter so as to capitalize on this advantage. However, in existing systems, including the one listed above, lock-up clutch control that adequately capitalizes on the advantage of a motor or motor generator is not necessarily always performed.